This invention relates to methods and systems for high-speed, 3D imaging of optically-invisible radiation and detectors and arrays of such detectors for use therein.
One of the fundamental problems involving work with radioactive materials is that radiation is invisible to the human eye and thus poses an invisible hazard. The hazard is compounded when one considers that these materials can be present in an environment when not expected such as with radioactive contamination or leaking radioactive waste storage tanks. To make the concern even more valid, these sources of radiation can be moving, as can be the case with airborne contamination. Thus, it is clear that there is a need for a way to localize radioactive sources, preferably in real-time.
Much work has been done on ways to image various forms of radiation to provide the user with a xe2x80x9cpicturexe2x80x9d of the radiation present in an environment. Currently available gamma-ray cameras are capable of providing two-dimensional information about the location and spectroscopy of a radioactive source similar to taking a snapshot with a standard camera. However, these cameras are not independently capable of providing information to locate the source in three dimensions. There have been cameras built that are capable of obtaining real-time information, which is useful for viewing changing sources. However, based on current designs, the performance of some tasks in radiation environments precludes simultaneous monitoring of the radiation field by the individual worker, possibly resulting in increased radiation exposures. 3D detection systems are available for medical and other environments, but these involve different geometries and source distributions than those considered here. Also, these methods rely on complex mathematical reconstruction making them cumbersome and time-consuming.
A new problem arises if one considers the complex environments that these sources can exist in since even when radiation images are blended with light images three-dimensionality is lost, real-time manipulation of the images becomes complex, and difficulties arise with time-varying source distributions. Only three-dimensional source location truly allows for accurate position determinations of radioactive materials. Furthermore, real-time simultaneous display of the physical and radiation environments is essential for observing moving or redistributing radiation sources.
Augmented Reality
Both virtual reality (VR) and augmented reality (AR) provide real-time interactivity which requires 3D registration. VR and AR require a motion tracker to determine the user""s position in the virtual environment (VE), a computer to coordinate the user""s relative location, and a display. VR and AR are currently being used in various fields including research and development, design and testing, navigation and targeting, training, and visualization (Azuma, 1997). There exists a wide variety of hardware and software capable of displaying VEs. Virtual Reality Modeling Language (VRML) 2.0 is the current industry standard for programming with many large software packages, such as AutoCAD and 3D Studio Max (Autodesk, Inc.), exporting to this file format. The display of VR is achieved by a head-mounted device (HMD), head-coupled display (HCD), or a Cave Automatic Virtual Environment (CAVE). AR display is limited to HMDs with modifications that allow the user to see the real world through the display.
With an VE application, there are always certain limitations that current researchers are trying to overcome. Those who program for VR or AR applications must achieve a high level of realism while not slowing down the computer system to intolerable speeds. Designers of VR and AR hardware must always consider problems arising from concerns of simplicity, spatial resolution, and safety. For AR, one must also be concerned with using reasonable separation for data collection and display so as to simulate the user""s interpupillary distance. Focus also presents a current field of AR research since the human eye, when observing real objects, must match virtual object focus at the same distance as the physical objects. Finally, current research is being conducted into how to increase the field of view of HMDs and HCDs to most accurately match that of the user (Azuma, 1997).
Semiconductor Technology
Semiconductor devices typically operated by measuring the number of electrons and holes excited by ionizing radiation (gamma rays or charged particles) within the detector. The number of excited charge carriers is remarkably linear with respect to the absorbed energy from an ionizing event. The excited charge carriers are drifted across the semiconductor detector by an externally applied electric field, which, in turn, produces an image charge or induced charge on the output circuit. Electrons are drifted toward the device anode and holes are drifted toward the device cathode. For a planar detector, the Shockley-Ramo (Shockley, 1938; Ramo, 1939) theorem describes the relationship between the induced charge (Q*) and the displacement distance of the free electrons and holes:                               Q          *                =                              Q            0                    ⁢                                                    "LeftBracketingBar"                                  Δ                  ⁢                                      xe2x80x83                                    ⁢                                      x                    e                                                  "RightBracketingBar"                            +                              "LeftBracketingBar"                                  Δ                  ⁢                                      xe2x80x83                                    ⁢                                      x                    h                                                  "RightBracketingBar"                                                    W              D                                                          (        1        )            
where Q0 is the initial magnitude of free charge liberated, xcex94x refers to the distance traveled by the electrons or holes from their point of origin toward their respective electrode, WD is the width of the planar detector, and e and h subscripts refer to electrons and holes, respectively. If the charge carriers are removed completely from the device, in which case they reach their respective electrodes, then the solution to Equation (1) is simply Q*=Q0. The importance of this result is that gamma-ray spectroscopy can be performed by simply measuring the total induced charge measured from electrons and holes drifted to the detector electrodes. In the presence of charge carrier trapping (caused by imperfections in the semiconductor), charge carriers often do not reach their respective electrodes, and the induced charge observed becomes very dependent on the location of the gamma-ray interaction (Day, Dearnaley and Palms, 1967; Knoll and McGregor, 1993). The Hecht relationship (Hecht, 1932) describes the expected induced charge for a planar detector with charge trapping:                               Q          *                =                              Q            0                    ⁢                      {                                                            ρ                  e                                ⁡                                  (                                      1                    -                                          exp                      ⁡                                              [                                                                                                            x                              i                                                        -                                                          W                              D                                                                                                                                          ρ                              e                                                        ⁢                                                          W                              D                                                                                                      ]                                                                              )                                            +                                                ρ                  h                                ⁡                                  (                                      1                    -                                          exp                      ⁡                                              [                                                                              -                                                          x                              i                                                                                                                                          ρ                              h                                                        ⁢                                                          W                              D                                                                                                      ]                                                                              )                                                      }                                              (        2        )            
where xi represents the interaction location in the detector as measured from the cathode. The electron or hole carrier extraction factor (Knoll and McGregor, 1993) is described by:                               ρ                      e            ,            h                          =                                            v                              e                ,                h                                      ⁢                          τ                              e                ,                h                            *                                            W            D                                              (        3        )            
where xcexd is the charge carrier mobility and xcfx84* is the carrier mean free drift time before a trapping event occurs. As can be observed from Equations (2) and (3) the induced charge becomes a function of the interaction location within the detector. High xcfx81 values (above 50) for both electrons and holes are desirable for high resolution gamma-ray spectroscopy. Unfortunately, the value of xcfx81h for most compound semiconductors is generally much lower than the value of xcfx81e. Largely differing values of xcfx81 for electrons and holes are not conducive to high resolution gamma-ray energy spectroscopy when using simple planar semiconductor detector designs (Day, Dearnaley and Palms, 1967; Knoll and McGregor, 1993).
Recent results with novel geometrically weighted Frisch grid CdZnTe detectors demonstrate dramatic improvements in gamma-ray resolution. The devices no longer require signals from hole transport, hence the higher carrier extraction factor values of the electrons can be manipulated while ignoring the difficulties imposed by hole trapping. The device uses the geometric weighting effect, the small pixel effect and the Frisch grid effect to produce high gamma-ray energy resolution. The design is simple and easy to construct. The device performs as a gamma-ray spectrometer without the need for pulse shape rejection or correction, and it requires only one signal output to any commercially available charge sensitive preamplifier. The device operates very well with conventional NIM electronic systems. Presently, room temperature (23xc2x0 C.) energy resolutions of 2.68% FWHM at 662 keV and 2.45% FWHM at 1.332 MeV have been measured with 1 cubic cm CdZnTe devices.
FIG. 5 shows the basic features of a geometrically weighted semiconductor Frisch grid radiation detector. The device dimensions are designated as follows: cathode width=Wc, anode width=Wa, width at the pervious region center=Wp, interaction region height=Li, pervious region height=Lp, measurement region height=Lm, overall detector height=H and the detector length=D. The major physical effects for the device are briefly discussed in the following sections.
For simplicity, one assumes that gamma-ray interactions occur uniformly throughout the detector volume. For a trapezoidal prism, the fraction of gamma-ray interactions occurring in the interaction region is approximated by:                                           F            i                    ≈                                                    (                                                      W                    c                                    +                                      W                    p                                                  )                            ⁢                              (                                                      2                    ⁢                                          L                      i                                                        +                                      L                    p                                                  )                                                    2              ⁢                              (                                                      W                    a                                    +                                      W                    c                                                  )                            ⁢                              (                                                      L                    i                                    +                                      L                    p                                    +                                      L                    m                                                  )                                                    ,                            (        4        )            
For the following examples, a restraint of Wa=2 mm is imposed in all cases. With Wc=10 mm, D=10 mm, H=10 mm, xcex8=43.5xc2x0 and with the Frisch grid=1 mm wide centered 2.0 mm back from the anode, the fraction of events occurring in the interaction region can be shown to be 85.3%. The overall result is high gamma-ray sensitivity in the interaction region and high rejection for gamma-ray interactions occurring in the measurement region while retaining good screening with the Frisch grid.
The gamma-ray interaction probability distribution function is highest near the cathode and lowest near the anode for a trapezoidal prism semiconductor Frisch grid detector. For uniform irradiation, the normalized total gamma-ray probability distribution function for a trapezoidal device is:                                                                         P                N                            ⁡                              (                x                )                                      ⁢                          ⅆ              x                                =                                                                      2                  ⁢                  x                  ⁢                                      xe2x80x83                                    ⁢                                      tan                    ⁡                                          (                                              θ                        2                                            )                                                                      +                                  W                  a                                                                                                  H                    2                                    ⁢                                      tan                    ⁡                                          (                                              θ                        2                                            )                                                                      +                                  HW                  a                                                      ⁢                          ⅆ              x                                      ,                  0          ≤          x          ≤          H                ,                            (        5        )            
where x refers to the distance from the anode toward the cathode and xcex8 refers to the acute angle at the anode (see FIG. 5). Returning to the previous example, consider the number of gamma-ray interactions that occur within 1 mm of the cathode. Integrating Equation (2) from x=9 mm to x=10 mm yields a normalized interaction probability of 16%, whereas integrating from x=0 mm to x=1 mm yields a normalized gamma-ray interaction probability of 3.9%. Hence, over four times as many events occur within 1 mm of the cathode than within 1 mm of the anode, which serves to demonstrate that the accumulated gamma-ray pulse height spectrum will be formed primarily from electron dominated induced charge pulses. The probability of electron-dominated induced charge motion is much higher than hole-dominated induced charge motion for simple geometric reasons.
The signal formation from a basic planar type semiconductor detector has a linear dependence between the carrier travel distance and the induced charge (Day, Dearnaley and Palms, 1967; Knoll and McGregor, 1993). Such a relationship is not true when the contacts of a device are not the same size (Shockley, 1938; Barrett, Eskin and Barber, 1995). The xe2x80x9csmall pixelxe2x80x9d effect is a unique weighting potential and induced charge dependence observed with devices having different sized electrodes (Barrett, Eskin and Barber, 1995).
In the case that a detector has a small anode and a large cathode, the weighting potential changes much more abruptly near the anode than the region near the cathode. As a result, more charge is induced as charge carriers move in the vicinity of the small anode than charge carriers moving in the vicinity near the cathode. From the natural effect of geometrical weighting, more charge carrier pairs are produced near the cathode over that of the anode. As a result, more electrons will be drifted to the region near the small anode than the number of holes xe2x80x9cbornxe2x80x9d at the small anode. The result is that the induced charge influenced by the electron carriers becomes even greater when the small pixel effect is coupled to the geometrically weighted effect. The combined effects of geometrical weighting and the small pixel effect cause the formation of a xe2x80x9cpseudo-peakxe2x80x9d, a peak that is gamma-ray energy dependent, but forms as a direct consequence of the geometrical shape of the device and the device electrodes.
Device performance is best with the Frisch grid turned on due to the hole charge motion screening (McGregor et al., 1999; McGregor and Rojeski, 1999; McGregor et al. 1998). The Frisch grid acts as the reference plane by which charge carriers induce charge on the anode. Only after electrons pass into the measurement region (see FIG. 5) do they begin to form an induced charge signal on the preamplifier. Since holes are moving in the opposite direction (toward the cathode), the difficulties imposed by hole trapping are significantly negated.
Charge carriers excited in the xe2x80x9cinteraction regionxe2x80x9d are drifted into a xe2x80x9cmeasurement regionxe2x80x9d. The measured induced charge begins to accumulate only when the free carriers enter into the measurement region, hence the device is designed such that carrier transport comes mainly from electrons moving into the interaction region.
Research has been undertaken in France to use AR for the teleoperation of robots in nuclear environments in order to develop safer and more efficient procedures for maintenance and dismantling (Viala and Letelleir, 1997). Telerobotics using AR is also being explored by research groups in the United States whose goal is to develop a semi-autonomous robot using a VE of the nuclear power plant being used (Rocheleau and Crane, 1991). The most pertinent research project whose purpose is to perform a radiological analysis by VR simulation for predicting radiation doses for robotic equipment working at the Hanford Site (Knight et al. 1997). The outcome of this research was to provide a static representation of radiation. Mapping vasculature at an angiographic level of detail is described by Bullitt et al. and Chen and Metz. However, 3D digital angiography involves relatively simple, string-like geometries which lend themselves to easy visualization using its method, and it also benefits from a fixed user position relative to the structures of interest.
U.S. Pat. No. 5,418,364 to Hale discloses an optically multiplexed dual line of sight system. Dual lines of sight pass through dual independent thermal references and produce two separate video signals, which can be viewed separately or simultaneously.
U.S. Pat. No. 4,931,653 to Hamm discloses an ionizing radiation detector system. The system determines the three-dimensional spatial distribution of all secondary electrons produced. A 3-D image is reconstructed by combining the digital images produced by video cameras. The system analyzes the electromagnetic spectrum from visible through gamma-ray radiation.
U.S. Pat. No. 4,957,369 to Antonsson discloses an apparatus for measuring three-dimensional surface geometries. A pair of diode detectors, mounted on the focal length of the cameras, reconstruct the full three-dimensional geometry of the surface examined using infrared radiation.
The following U.S. patents provide general background information: U.S. Pat. Nos. 3,932,861; 4,118,733; 4,868,652; and 5,534,694.
An object of the present invention is to provide a method and system for high-speed, 3D imaging of optically-invisible radiation and detector and array of such detectors for use therein wherein 3D radiation images are superimposed on a view of the environment.
In carrying out the above object and other objects of the present invention, a method is provided for high-speed, 3D imaging of optically-invisible radiation. The method includes detecting optically-invisible radiation within an environment to obtain signals and processing the signals to obtain stereoscopic data. The method also includes displaying the stereoscopic data in the form of optically-visible radiation images superimposed on a view of the environment so that a user can obtain a 3D view of the radiation by utilizing natural human stereo imaging processes.
The environment may be a virtual environment (i.e. generated using a computer or other means) or it may be an optically-visible (i.e. physical or real) environment.
The radiation may be ionizing radiation or may be infrared radiation. Ionizing radiation works to stimulate detectors; such radiation includes charged particles, electromagnetic waves and neutrons-sensitive coatings (like 9B, 6Li).
In further carrying out the above object and other objects of the present invention, a system is provided for high-speed, 3D imaging of optically-invisible radiation. The system includes a detector subsystem for detecting optically-invisible radiation within an environment to obtain signals and a signal processor for processing the signals to obtain stereoscopic data. The system also includes a display subsystem for displaying the stereoscopic data in the form of optically-visible radiation images superimposed on a view of the environment so that a user can obtain a 3D view of the radiation by utilizing natural human stereo imaging processes.
The detector subsystem may include a set of field detectors, a set of point detectors, a set of passive detectors, and/or a set of active detectors.
The radiation may be gamma-ray radiation wherein the set of field detectors includes a pair of gamma-ray cameras. The gamma-ray cameras may be scanning gamma-ray cameras wherein each of the gamma-ray cameras is capable of scanning the environment through a plurality of angles and wherein the signals are processed to locate a source within the environment.
The radiation may be ionizing radiation wherein the detector subsystem includes a scintillator and a collimator for directing the ionizing radiation into the scintillator or any other radiation detector which may be curved.
The detector subsystem may include a compound eye detector including a plurality of individual detectors. The plurality of individual detectors may be movable independently or as a group. The compound eye detector may include a single detector movable in three dimensions.
The signal processor may process the signals to obtain a 3D map of radiation-emitting sources.
The detector subsystem may have stereoscopic capabilities and may be portable.
The display subsystem may include a see-through display subsystem such as a screen which may be portable or head-mountable. The system may then include a tracking subsystem for tracking the display subsystem.
The system typically provides real-time visual feedback about the location and relative strength of at least one radiation-emitting source.
Still further in carrying out the above objects and other objects of the present invention, an ionizing radiation detector is provided. The detector includes an ionization substrate for converting ionizing radiation into a signal, a converter coupled to the substrate for converting the signal into a corresponding electrical signal, and a positioner for moving the substrate in three dimensions to image over a surface of a sphere.
The substrate may be a scintillator for converting ionizating radiation into photons of light. The signal is an optical signal and the converter may be a photodetector or a multiplier phototube.
Yet still further in carrying out the above objects and other objects of the present invention, an array of detectors is provided wherein each of the detectors is a detector as noted above. The detectors are arranged in a curvilinear geometry. For example, the detectors may be arranged so that the array forms a substantially hemispherical device.
Preferably, the substrates of the detectors are formed from separate materials.
Still further in carrying out the above objects and other objects of the present invention, an ionizing radiation detector is provided. The detector includes an ionization substrate formed from a single material. The substrate may have a curved first surface and a second surface opposing the first surface for converting ionizing radiation at the curved first surface into a signal. The detector also includes a radiation shield disposed at the second surface to substantially block ionizing radiation at the second surface.
The radiation shield may be a fanned collimator. The ionization substrate may be a curved scintillator for converting ionizating radiation into photons of light.
The ionization substrate may be a semiconductor substrate.
The detector may form a substantially hemispherical device.
Preferably, the second surface is curved and is substantially parallel to the curved first surface.
The method and system of the present invention have several unique benefits for potential users. In general, the invention has its strongest applications in dose minimization since it allows the user to see the radiation in the environment she is working in. For example, there are many instances when one desires to locate radioactive contamination in an environment. These environments can be quite complex thus requiring more sophisticated images than the standard 2D images. Contamination searches are presently conducted by a radiation worker with a survey meter who spends a great deal of time inspecting the environment by hand. The invention would allow the user to obtain rapid 3D radiation maps in real-time. Should the source by moving or changing, this would be able to be monitored. Thus, the clean up of the contamination would be significantly faster, reducing the worker""s exposure to the radiation. This application would be extremely useful to any industrial or laboratory setting which uses gamma-ray radiation.
Another example involves the survey of waste drums or casks such as those stored at Hanford National Laboratory (HNL), a facility run by the Department of Energy. Such containers require constant monitoring to determine if they are leaking. This monitoring could be quickly and easily achieved by the invention which would minimize worker time and possible exposures to unnecessary amounts of radiation.
These casks at HNL and similar casks and waste drums would provide another interesting problem that the invention could solve. It is frequently the case that little is known about the isotropic concentration of materials within the containers. For example, the HNL casks are a sludge of various radioisotopes, but little is known about where within the cask each isotope is located. It is also possible that there could be various types of solid waste within a waste drum, but its position and orientation within the drum is not known. Using its spectroscopic features, the method and system of the invention can select an energy region of interest and image just materials emitting that particular energy, thus determining the position within the drum or cask of materials of the isotope in question.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.